Most enemy spell casters have a rank of Lieutenant or less. Crushing Prison will disable them for its duration.
The spell is not very effective against bosses, as they are not immobilized and damage is scaled down with the very low duration. One notable exception is when tanking with an Arcane Warrior, where the immediate extra threat is handy to draw the boss to the tank.
Unlike Force Field, enemies in Crushing Prison will continue to take damage. This spell can be very effective if combined with Area of Effect spells such as Inferno.
It is possible to target friendlies with crushing prison, allowing to trigger Shockwave around friendly targets when combining it with Force Field.